1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine and a printer, in which a toner image formed on an image bearing member is transferred onto an intermediate transfer belt, and is subsequently transferred to a recording material.
2. Description of the Related Art
Conventionally, as an image forming apparatus such as a copying machine and a laser beam printer, a configuration which includes an intermediate transfer belt as an intermediate transfer member is known. In this image forming apparatus, toner images formed on image bearing members of respective image forming portions are transferred onto an intermediate transfer belt by a primary transfer process so as to be superposed, and are subsequently transferred collectively from the intermediate transfer belt onto a recording material by a secondary transfer process so as to form a color image (multiple images).
The image forming portions of respective colors include photosensitive drums as the image bearing members, respectively. Moreover, each image forming portion includes, in addition to the above-mentioned photosensitive drum, a charging member which charges the photosensitive drum, and a developing unit which develops the toner image on the photosensitive drum. The charging member of each image forming portion comes into contact with the photosensitive drum under a predetermined contact pressure, so as to uniformly charge the surface of the photosensitive drum with a predetermined polarity and potential by a charging voltage which is applied from a voltage power supply (not shown) for charging.
The toner image developed on the photosensitive drum of each image forming portion is primarily transferred onto the intermediate transfer belt by a primary transfer member which is opposed to the photosensitive drum across the intermediate transfer belt. A power supply for transfer (power circuit), which is dedicated to the primary transfer, is connected to each primary transfer member. The toner image primarily transferred onto the intermediate transfer belt is secondarily transferred onto the recording material by a secondary transfer member. A power supply for transfer (power circuit), which is dedicated to the secondary transfer, is connected to the secondary transfer member.
For example, Japanese Patent Application Laid-Open No. 2003-195697 discloses that four power supplies for transfer used for the primary transfer are required for the primary transfer for the respective colors, and further, a single power supply for transfer used for the secondary transfer is required for the secondary transfer. These power supplies for transfer used for the primary transfer are required to apply a voltage, which is set so as to obtain an optimum ability of primary transfer, to the primary transfer members. When a large number of power supplies for transfer used for the primary transfer are required, a large number of supply portions of high voltage output are disposed on a high voltage substrate, so that the high voltage substrate is inevitably enlarged.
Moreover, in a configuration in which an intermediate transfer unit can be removed from an apparatus main body of an image forming apparatus so as to be exchanged, when the number of the power supplies for transfer dedicated to the primary transfer is large, a large number of electric contacts are required between the intermediate transfer unit and the apparatus main body. Accordingly, there are concerns that the apparatus may be complicated, and the cost may be increased due to the increase of the number of parts.